1. Field of the Invention
This invention relates, in general, to three state gates and, more particularly, to an inverting three state gate wherein the transition from an active high to an active low output is enhanced.
2. Background Art
Three state gates have an output which is capable of assuming an active high, an active low, or a high impedance state. Generally, most previously known gates having bipolar transistors comprise a push-pull output driver stage, a phase-splitting stage, and an input stage. The push-pull output driver stage comprises a dual transistor arrangement wherein an upper transistor is coupled between a DC voltage supply and an output load and a lower transistor is coupled between the output load and ground. In operation, a high output voltage is realized at the output terminal by turning on the upper transistor and turning off the lower transistor; a low output voltage is realized by turning off the upper transistor and turning on the lower transistor; and a high impedance is achieved by turning off both transistors.
The phase splitting stage comprises a transistor coupled between the bases of the two output transistors of the output stage that would selectively turn on one of the two output stage transistors. The input stage comprises a PNP transistor having a base connected to an input terminal. A first NPN transistor has its base connected to the emitter of the PNP transistor. A second NPN transistor has a base connected to the emitter of the first NPN transistor. The bases of the first NPN transistor, the second NPN transistor, the phase splitting transistor, and the output stage upper transistor are each coupled to an output enable terminal by diodes. A low output enable signal directs current away from the transistor bases through these diodes, thus turning off both of the upper and lower transistors of the output stage, giving a high impedance at the output terminal. A high signal on the output enable terminal reverse biases the diodes, effectively removing their paths from the circuit.
It is desired to have the output transition rapidly from an active high to an active low when the appropriate input signal is received. One previously known method of increasing the downward transition of the output signal is shown in FIG. 1 and will be described in detail in the Detailed Description of the Invention. Generally, a feedback diode is coupled between the output terminal and the collector of a feedback transistor for momentarily conducting current away from the output terminal until an active low is reached wherein the first diode will be reverse biased. A second feedback diode is coupled between the base of the upper output transistor and the phase splitting means for momentarily conducting current in a similar fashion to drain the capacitive charge on the base-collector of the upper output transistor.
A second previously known method of enhancing the transition from an active high to an active low of the output is shown in FIG. 2 and is also described in detail in the Detailed Description of the Invention. Generally, a feedback transistor is coupled between the output transistor and the phase splitting means and has its base connected to the base of the upper output transistor. A first diode is coupled between the feedback transistor and the output terminal. A second diode is coupled between the base of the upper output transistor and the output enable terminal. This arrangement again enhances the transition of the output from an active high to an active low by momentarily pulling current away from the output terminal and also discharging the capacitive charge stored on the base-collector of the upper output transistor.
However, the circuit as shown in FIG. 1 requires matching of a transistor in the phase splitting means and the feedback transistor to avoid current hogging of their respective base drive. Furthermore, none of the previously known three state gates provide a method of directly controlling the speed of the enhanced transition to an active low state. Furthermore, the previously known circuits do not compensate for reflected waves (i.e., transmission lines) when in the active low state.
Thus, a need exists for an improved three state gate having an enhanced transition to an active low wherein the speed of transition may be directly controlled, the matching of devices is eliminated, and which compensates for reflected waves at the output terminal in the active low state.